As the air's CO2 content continues to rise, it is important to determine what effect this phenomenon might have on the delicate balance that exists between various plants and the insects that feed on them. We have treated this subject in some detail with respect to aphids, butterflies and moths in other sections of our website [see Insects (Aphids) and Insects (Butterflies and Moths) in our Subject Index]. Here, we review the results of studies that have reported on other insects.
Docherty et al. (1997), in addition to studying aphids, studied two sap-feeding leafhopper species that were allowed to feed on saplings of beech and sycamore that were grown in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm. As far as they could determine, there were no significant effects of the extra CO2 on either the feeding or performance characteristics of either leafhopper species.
In a literature review of more than 30 studies published two years later, Whittaker (1999) found that chewing insects (leaf chewers and leaf miners) showed either no change or actual reductions in abundance in response to atmospheric CO2 enrichment, noting, however, that population reductions in this feeding guild were often accompanied by increased herbivory in response to CO2-induced reductions in leaf nitrogen content.
Contemporaneously, Stiling et al. (1999) reported the results of what may possibly have been the first attempt to study the effects of elevated CO2 on trophic webs in a natural ecosystem, specifically, a nutrient-poor scrub-oak community in Florida, USA, where sixteen open-top chambers of 3.6-m diameter were fumigated with air of either 350 or 700 ppm CO2 for approximately one year. At the end of this period, total leaf miner densities were 38% less on the CO2-enriched foliage than on the foliage of the ambiently-grown plants. Moreover, atmospheric CO2 enrichment consistently reduced the numbers of all six species of leaf miners studied. In a compensatory development, however, exposure to elevated CO2 increased the amount of leaf area consumed by the less abundant leaf miners by approximately 40%. Nevertheless, leaf miners in the CO2-enriched chambers experienced significantly greater mortality than those in the control chambers. Although CO2-induced reductions in leaf nitrogen content were determined to have played a minor role in the mortality increase, the greatest factor contributing to this phenomenon was a four-fold increase in parasitization by various wasps that could more readily detect the more-exposed leaf miners on the CO2-enriched foliage.
Three years later, Stiling et al. (2002) reported even more dramatic effects of the elevated CO2 of this experiment on leaf chewers. The relative levels of damage by these insects (primarily larval lepidopterans and grasshoppers) were significantly lower in the elevated CO2 chambers than in the ambient CO2 chambers for all five of the plant species that accounted for over 98% of the total plant biomass of the ecosystem. In addition, the response to elevated CO2 was the same across all plant species. Also, they reported that more host-plant-induced mortality was found for all miners on all plants in elevated CO2 than in ambient CO2. These effects were so powerful, in fact, that in addition to the relative densities of insect herbivores being reduced in the CO2-enriched chambers, and even though there were more leaves on most plant species in the elevated CO2 chambers, the total densities of leaf miners in the high-CO2 chambers were also lower for all plant species. An interesting implication of these findings, as expressed by Stiling et al., is that "reductions in herbivore loads in elevated CO2 could boost plant growth beyond what might be expected based on pure plant responses to elevated CO2," which is truly an exciting thing to contemplate.
In another experiment conducted on a natural ecosystem in Wisconsin, USA -- comprised predominantly of trembling aspen (Populus tremuloides Michx.) -- Percy et al. (2002) studied the effects of increases in CO2 alone (to 560 ppm during daylight hours), O3 alone (to 46.4-55.5 ppb during daylight hours), and CO2 and O3 together on the forest tent caterpillar (Malacosoma disstria), a common leaf-chewing lepidopteran found in North American hardwood forests. By itself, elevated CO2 reduced caterpillar performance by reducing female pupal mass; while elevated O3 alone improved caterpillar performance by increasing female pupal mass. When both gases were applied together, however, the elevated CO2 completely counteracted the enhancement of female pupal mass caused by elevated O3. Hence, either alone or in combination with undesirable increases in the air's O3 concentration, elevated CO2 tended to reduce the performance of the forest tent caterpillar. This finding is particularly satisfying because, in the words of Percy et al., "historically, the forest tent catepillar has defoliated more deciduous forest than any other insect in North America," and because "outbreaks can reduce timber yield up to 90% in one year, and increase tree vulnerability to disease and environmental stress."
In a study of yet another type of insect herbivore, Brooks and Whittaker (1999) removed grassland monoliths from the Great Dun Fell of Cumbria, UK -- which contained eggs of a destructive xylem-feeding spittlebug (Neophilaenus lineatus) -- and grew them in glasshouses maintained for two years at atmospheric CO2 concentrations of 350 and 600 ppm. During the course of their experiment, two generations of the xylem-feeding insect were produced; and in each case, elevated CO2 reduced the survival of nymphal stages by an average of 24%. Brooks and Whittaker suggest that this reduction in survival rate may have been caused by CO2-induced reductions in stomatal conductance and transpirational water loss, which may have reduced xylem nutrient-water availability. Whatever the mechanism, the results of this study bode well for the future survival of these species-poor grasslands as the air's CO2 content continues to rise.
Finally, we report one last study that has nothing to do with CO2 directly, but with global warming, which climate-alarmists claim is caused by elevated levels of atmospheric CO2. In this work of Thomas et al. (2001), it is demonstrated that, in response to regional warming in the British Isles over the last two decades of the 20th century, there was an unusually rapid northward expansion of the ranges of two species of cricket: the long-winged cone-head (Conocephalus discolor) and Roesel's bush cricket (Metrioptera roeselii). The reason for this increased rate of range expansion, according to Thomas et al., resides in the fact that, in response to the regional warming, the crickets produced "increased fractions of longer-winged (dispersive) individuals in recently founded populations" that "resulted in about 3- to 15-fold increases in expansion rates, allowing these insects to cross habitat disjunctions that would have represented major or complete barriers to dispersal before the expansions started."
In summing up the implications of the various phenomena described in this Subject Index Summary, it would appear that both CO2-induced and warming-induced changes in the physical characteristics and behavioral patterns of a diverse assemblage of insect types portend good things for the biosphere in the years and decades to come.
Brooks, G.L. and Whittaker, J.B. 1999. Responses of three generations of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2. Global Change Biology 5: 395-401.
Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J. 1997. Responses of tree sap-feeding herbivores to elevated CO2. Global Change Biology 3: 51-59.
Percy, K.E., Awmack, C.S., Lindroth, R.L., Kubiske, M.E., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F. 2002. Altered performance of forest pests under atmospheres enriched by CO2 and O3. Nature 420: 403-407.
Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G. 2002. Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia DOI 10.1007/s00442-002-1075-5.
Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B. 1999. Decreased leaf-miner abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack. Ecological Applications 9: 240-244.
Thomas, C.D., Bodsworth, E.J., Wilson, R.J., Simmons, A.D., Davies, Z.G., Musche, M. and Conradt, L. 2001. Ecological and evolutionary processes at expanding range margins. Nature 411: 577-581.
Whittaker, J.B. 1999. Impacts and responses at population level of herbivorous insects to elevated CO2. European Journal of Entomology 96: 149-156.